Trench isolation structures and methods for bipolar junction transistors

ABSTRACT

Device structures, fabrication methods, and design structures for a bipolar junction transistor. A first isolation region is formed in a substrate to define a lateral boundary for an active device region and an intrinsic base layer is formed on the substrate. The intrinsic base layer has a section overlying the active device region. After the intrinsic base layer is formed, the first isolation region is partially removed adjacent to the active device region to define a trench that is coextensive with the substrate in the active device region and that is coextensive with the first isolation region. The trench is at least partially filled with a dielectric material to define a second isolation region.

BACKGROUND

The present invention relates generally to semiconductor devicefabrication and, in particular, to device structures for a bipolarjunction transistor, fabrication methods for a bipolar junctiontransistor, and design structures for a bipolar junction transistor.

Bipolar junction transistors are typically found in demanding types ofintegrated circuits, especially integrated circuits destined forhigh-frequency applications and high-power applications. One specificapplication for bipolar junction transistors is in radiofrequencyintegrated circuits (RFICs), which are found in wireless communicationssystems, power amplifiers in cellular telephones, and other varieties ofhigh-speed integrated circuits. Bipolar junction transistors may also becombined with complementary metal-oxide-semiconductor (CMOS) fieldeffect transistors in bipolar complementary metal-oxide-semiconductor(BiCMOS) integrated circuits, which take advantage of the positivecharacteristics of both transistor types in the construction of theintegrated circuit.

Conventional bipolar junction transistors constitute three-terminalelectronic devices constituted by three semiconductor regions, namely anemitter, a base, and a collector. An NPN bipolar junction transistorincludes two regions of n-type semiconductor material constituting theemitter and collector, and a region of p-type semiconductor materialsandwiched between the two regions of n-type semiconductor material toconstitute the base. A PNP bipolar junction transistor has two regionsof p-type semiconductor material constituting the emitter and collector,and a region of n-type semiconductor material sandwiched between tworegions of p-type semiconductor material to constitute the base.Generally, the differing conductivity types of the emitter, base, andcollector form a pair of p-n junctions, namely a collector-base junctionand an emitter-base junction. A voltage applied across the emitter-basejunction of a bipolar junction transistor controls the movement ofcharge carriers that produce charge flow between the collector andemitter regions of the bipolar junction transistor.

Improved device structures, fabrication methods, and design structuresare needed that enhance the device performance of bipolar junctiontransistors.

BRIEF SUMMARY

In an embodiment of the invention, a method is provided for fabricatinga bipolar junction transistor. The method includes forming a firstisolation region in a substrate to define a lateral boundary for anactive device region and forming an intrinsic base layer on thesubstrate. The intrinsic base layer has a section overlying the activedevice region. After the intrinsic base layer is formed, the firstisolation region is partially removed adjacent to the active deviceregion to define a trench that is coextensive with the substrate in theactive device region and that is coextensive with the first isolationregion. The method further includes at least partially filling thetrench with a dielectric material to define a second isolation region.

In an embodiment of the invention, a device structure is provided for abipolar junction transistor. The device structure includes a firstisolation region defining a boundary of an active device region in thesubstrate, an intrinsic base on the active device region of thesubstrate, a collector in the active device region of the substrate, anemitter coupled by the intrinsic base with the collector, and a secondisolation region at least partially inside the first isolation region.The second isolation region is coextensive with the first isolationregion and is coextensive with the active device region of thesubstrate.

In an embodiment of the invention, a hardware description language (HDL)design structure is encoded on a machine-readable data storage medium.The HDL design structure comprises elements that, when processed in acomputer-aided design system, generates a machine-executablerepresentation of a bipolar junction transistor. The HDL designstructure includes a first isolation region defining a boundary of anactive device region in a substrate, an intrinsic base on the activedevice region of the substrate, a collector in the active device regionof the substrate, an emitter coupled by the intrinsic base with thecollector, and a second isolation region at least partially inside thefirst isolation region. The second isolation region is coextensive withthe first isolation region and is coextensive with the active deviceregion of the substrate. The HDL design structure may comprise anetlist. The HDL design structure may also reside on storage medium as adata format used for the exchange of layout data of integrated circuits.The HDL design structure may reside in a programmable gate array.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIGS. 1-8 are cross-sectional views of a portion of a substrate atsuccessive fabrication stages of a processing method for fabricating adevice structure in accordance with an embodiment of the invention.

FIGS. 4A, 4B, 4C are a cross-sectional views similar to FIG. 4 inaccordance with alternative embodiments of the invention.

FIG. 5A is a cross-sectional view similar to FIG. 5 in accordance withan alternative embodiment of the invention

FIG. 9 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with an embodiment of theinvention, a substrate 10 may be any suitable bulk substrate comprisinga semiconductor material that a person having ordinary skill in the artwould recognize as suitable for forming an integrated circuit. Forexample, substrate 10 may be comprised of a wafer of a single crystalsilicon-containing material, such as single crystal silicon with a (100)crystal lattice orientation. The semiconductor material comprisingsubstrate 10 may be lightly doped with an impurity to alter itselectrical properties and may also include an optional epitaxial layer.

Trench isolation regions 12, 13 that surround and electrically isolatean active device region 14 of the substrate 10, which is used in thefabrication of a bipolar junction transistor 70 (FIG. 8). The trenchisolation regions 12, 13 may be formed in the substrate 10 by a shallowtrench isolation (STI) technique that deposits a hardmask, definetrenches in hardmask and substrate 10 with lithography and dry etchingprocesses, deposits an electrical insulator to fill the trenches,planarizes the electrical insulator relative to the hardmask using achemical mechanical polishing (CMP) process, and removes the hardmask.The electrical insulator constituting the trench isolation regions 12,13 may be comprised of an oxide of silicon (e.g., silicon dioxide(SiO₂)) deposited by chemical vapor deposition (CVD).

The trench isolation regions 12, 13 are coextensive with the activedevice region 14 along respective vertical interfaces 16, 17, which arenominally co-planar with sidewalls of the trenches used to form thetrench isolation regions 12, 13 and define respective lateral boundariesof the active device region 14. Corners of the trench isolation regions12, 13 and active device region 14 adjoin at the top surface 10 a andare aligned with the vertical interfaces 16, 17.

The trench isolation regions 12, 13 have respective top surfaces 12 a,13 a that may be nominally coplanar with the top surface 10 a of thesubstrate 10 in the active device region 14. Divots 15 appear in thetrench isolation regions 12, 13 proximate to the corners and alter theplanarity of the top surfaces 12 a, 13 a. The divots 15 operate toreduce the degree of planarity between the top surface 10 a and the topsurfaces 12 a, 13 a.

A collector 18 is disposed in the active device region 14 and asubcollector 20 is disposed in the substrate 10 beneath the collector18. The collector 18 and subcollector 20 are constituted by dopedregions of semiconductor material having the same conductivity type. Atop surface of the collector 18 may be coextensive with the top surface10 a of the substrate 10 in active device region 14. The collector 18 ispositioned interior of the trench isolation regions 12, 13. Thesubcollector 20 extends from the collector 18 beneath the trenchisolation regions 12, 13 to a reach-through region 21 that is exteriorof the trench isolation regions 12, 13. The reach-through region 21facilitates the establishment of external electrical contact with thecollector 18 and subcollector 20. The collector 18 and subcollector 20may comprise an electrically-active dopant, such as an n-type impurityspecies from Group V of the Periodic Table (e.g., phosphorus (P),arsenic (As), or antimony (Sb)) that is effective to impart n-typeconductivity in silicon.

An intrinsic base layer 22 is formed as a continuous additive layer onthe top surface 10 a of the substrate 10 in active device region 14. Theintrinsic base layer 22 is coupled with the collector 18 and maydirectly contact the collector 18. A patterned screen layer 23 may beapplied and opened proximate to the active device region 14. Thepatterned screen layer 23, which may be comprised of SiO₂, covers thetop surface 10 a at the reach-through region 21.

The intrinsic base layer 22 may be comprised of a semiconductormaterial, such as silicon-germanium (SiGe) including silicon (Si) andgermanium (Ge) in an alloy with the silicon content ranging from 95atomic percent to 50 atomic percent and the germanium content rangingfrom 5 atomic percent to 50 atomic percent. The germanium content of theintrinsic base layer 22 may be uniform or the germanium content ofintrinsic base layer 22 may be graded and/or stepped across thethickness of intrinsic base layer 22. If the germanium content isstepped, a thickness of the intrinsic base layer 22, such as thicknessesdirectly contacting the collector 18 or directly adjacent to thesubstrate 10, may lack germanium and may instead be entirely comprisedof silicon. The intrinsic base layer 22 may be doped with one or moreimpurity species, such as boron, and may further include carbon tosuppress the outdiffusion of the impurity species.

Intrinsic base layer 22 may be formed using an epitaxial growth process,such as vapor phase epitaxy. Single crystal semiconductor material(e.g., single crystal silicon and/or single crystal silicon-germanium)is epitaxially grown in a section of intrinsic base layer 22 on the topsurface 10 a of the substrate 10 in active device region 14. Thesingle-crystal semiconductor material of the active device region 14serves as a crystalline template for the growth of the single crystalsection 24 of intrinsic base layer 22 that is coextensive with theactive device region 14.

The intrinsic base layer 22 further includes facets 26 that, in therepresentative embodiment, surround the single crystal section 24 and afield section 25. Due to the growth process, the single crystal section24 is aligned with the active device region 14 and the collector 18 inthe active device region 14. The facets 26 terminate at a locationnominally coinciding with (i.e., coterminous with) the verticalinterfaces 16, 17 respectively between the trench isolation regions 12,13 and active device region 14 and represent a transition to the fieldsection 25 of intrinsic base layer 22. The facets 26 of intrinsic baselayer 22 may be a comprised of a mixture of polycrystalline and singlecrystal semiconductor material. In contrast to the single crystalsection 24 and facets 26, the field section 25 of intrinsic base layer22 is typically comprised of polycrystalline semiconductor material thatmay be discontinuous and non-uniform.

A hardmask 28 is formed on a top surface 22 a of the intrinsic baselayer 22. In the representative embodiment, the hardmask 28 is comprisedof a layered combination of materials, such as a layer 29 of siliconnitride (Si₃N₄) and a thinner layer 27 of SiO₂ between the Si₃N₄ layerand the top surface 22 a of the intrinsic base layer 22. The materialsof layers 27, 29, which may be formed by wet or dry thermal oxidation,CVD, or a combination of these processes, may be selected to etchselectively to the semiconductor material of intrinsic base layer 22 andto be readily removed at a subsequent fabrication stage.

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage of theprocessing method, a mask layer 30 is applied on the top surface of thehardmask 28 and used to pattern the hardmask 28. The mask layer 30 maycomprise a photoresist that is applied with a spin coating process,pre-baked, exposed to a radiation projected through a photomask, bakedafter exposure, and developed with a chemical developer to define apattern with openings 32, 34 coinciding with the intended locations oftrenches extending through the intrinsic base layer 22 and into thetrench isolation regions 12, 13. The openings 32, 34 may be transferredby one or more etching processes from the mask layer 30 to the layers27, 29 of hardmask 28 and stopping on the material of the intrinsic baselayer 22. Each etching process may rely on a given etch chemistry, andeach etching process may comprise a wet chemical etch or a dry etch.

The openings 32, 34 in the hardmask 28 and mask layer 30 have innersidewalls 31 a, 33 a, which may be laterally located with reference tothe respective vertical interfaces 16, 17 along which the trenchisolation regions 12, 13 and active device region 14 are coextensive. Inone embodiment, the inner sidewall 31 a of opening 32 may be coplanarwith the vertical interface 16, and the inner sidewall 33 a of opening34 may be coplanar with the vertical interface 17. The location of theinner sidewalls 31 a, 33 a may be moved by adjusting the width, x, ofthe openings 32, 34 in the mask layer 30. For example, the width, x, maybe increased to shift the inner sidewalls 31 a, 33 a inwardly so thatthe inner sidewalls 31 a, 33 a are located inside the verticalinterfaces 16, 17. Alternatively, the width, x, may be maintainedconstant by shifting the location of the outer sidewalls 31 b, 33 binward as the inner sidewalls 31 a, 33 a are shifted inward.Alternatively, the openings 32, 34 may have different widths and thedistance from the inner sidewall 31 a to vertical interface 16 maydiffer from the distance from the inner sidewall 33 a to verticalinterface 17. The divots 15 in the trench isolation regions 12, 13 maybe exposed within the openings 32, 34 and therefore not covered by thehardmask 28.

The mask layer 30 is removed after the openings 32, 34 are extended intothe hardmask 28, followed by a cleaning process. If comprised of aphotoresist, the mask layer 30 may then be removed by ashing or solventstripping.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage of theprocessing method, trenches 36, 38 are formed using the patternedhardmask 28 as an etch mask. The trenches 36, 38 are aligned with theopenings 32, 34 in hardmask 28. An etching process is applied to removethe material of the intrinsic base layer 22 selective to the materialsof the hardmask 28 and trench isolation regions 12, 13. For example, theetching process may comprise reactive-ion etching (RIE) that producessidewalls for the trenches 36, 38 that are nominally perpendicular tothe top surface 10 a. The openings 32, 34 in the mask layer 30 may bepositioned relative to the intrinsic base layer 22 such that a portionof the field section 25 is removed but the single crystal section 24 andthe facets 26 of intrinsic base layer 22 remain intact. Alternativelyand as discussed above, the openings 32, 34 in the hardmask 28 may beshifted slightly inward so that the facets 26 are partially orcompletely removed by the formation of the trenches 36, 38.

Dielectric spacers 50, 51 may be formed on the intrinsic base layer 22and hardmask 28 bordering the trenches 36, 38 by depositing a conformallayer comprised of an electrical insulator, such as Si₃N₄ deposited byCVD, and shaping the conformal layer with an anisotropic etchingprocess, such as RIE, that preferentially removes the electricalinsulator from horizontal surfaces. The dielectric spacers 50, 51 maydirectly contact the respective top surfaces 12 a, 13 a of the trenchisolation regions 12, 13.

The trenches 36, 38 are then respectively extended into the trenchisolation regions 12, 13 with an etching process that removes thedielectric material of the trench isolation regions 12, 13 selective tothe materials of the hardmask 28, intrinsic base layer 22, substrate 10,and dielectric spacers 50, 51. The etching process may comprise a wetchemical etch or a dry etch. In one embodiment, the etching process maycomprise RIE to produce sidewalls 40, 42 bordering the substrate 10 inactive device region 14 and sidewalls 41, 43 bordering the trenchisolation regions 12, 13 that are nominally vertical relative to the topsurface 10 a.

The trench isolation regions 12, 13 are only partially removed by theetching process. The trenches 36, 38 penetrate into, but not through,the trench isolation regions 12, 13. To that end, the etching processmay be timed to stop before the trenches 36, 38 have advanced completelythrough the trench isolation regions 12, 13 and into the substrate 10beneath the trench isolation regions 12, 13. The area of trench 36 issmaller than the area of surface 12 a and the area of trench 38 issmaller than the area of surface 13 a viewed in a direction normal tothe surfaces 12 a, 13 a. The formation of the trenches 36, 38 may removethe divots 15 from the trench isolation regions 12, 13.

When complete, trenches 36, 38 extend from the top surface 22 a of theintrinsic base layer 22 through the intrinsic base layer 22 and into thetrench isolation regions 12, 13. The trenches 36, 38 may be connectedand surround the perimeter of the active device region 14.Alternatively, the trenches 36, 38 may be disconnected at the ends (notshown) of the active device region 14 in a direction normal to thestructure cross-section shown in FIG. 3. Trench 36 includes sidewalls40, 41 and a base 36 a connecting the sidewalls 40, 41. Trench 38includes sidewalls 42, 43 and a base 38 a connecting the sidewalls 42,43. Sidewall 40 is nominally coplanar with sidewall 31 a of opening 32and with the vertical interface 16. Sidewall 42 is nominally coplanarwith sidewall 33 a of opening 34 and with the vertical interface 17.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage of theprocessing method, the trenches 36, 38 are modified to laterally recessthe sidewalls 40, 42 into the substrate 10 within the active deviceregion 14, which alters the profile of the trenches 36, 38. The alteredprofile includes lateral extensions 44, 45 that undercut the singlecrystal section 24 of intrinsic base layer 22. The lateral extension 44of trench 36 is bounded by the sidewall 40 in substrate 10 within theactive device region 14, the bottom surface of the single crystalsection 24, and the base 36 a. The lateral extension 45 of trench 38 isbounded by the sidewall 42 in substrate 10 within the active deviceregion 14, the bottom surface of the single crystal section 24, and base38 a.

The sidewall 40 is sloped or inclined at an angle relative to the topsurface 10 a and undercuts the intrinsic base layer 22 beneath thesingle crystal section 24. The degree of the undercutting can becharacterized by a numerical bias or distance, w₁, measured between thesidewall 40 and the vertical interface 16. The sidewall 42 of trench 38is likewise sloped or inclined at an angle relative to the top surface10 a and undercuts the intrinsic base layer 22 beneath the singlecrystal section 24. The degree of the undercutting can be characterizedby a numerical bias or distance, w₂, measured between the sidewall 40and the vertical interface 17.

The undercutting may be uniform and symmetrical, in which instance thelateral extensions 44, 45 may have the same depth, width, and shape andthe inclination angles are the same. The symmetrical undercut istolerant of misalignment of the openings 32, 34 in the hardmask 28 andmask layer 30 (and hence the location of the trenches 36, 38 prior tothe etching process providing the undercutting) in that the openings 32,34 may be asymmetrically positioned relative to the single crystalsection 24 of intrinsic base layer 22 and/or have different widths. Theundercutting reduces the area of direct contact between the collector 18and the single crystal section 24 of intrinsic base layer 22.

To provide the lateral extensions 44, 45, the semiconductor material ofthe substrate 10 may be etched by a wet chemical etching process, a dryetching process, or a combination of wet chemical and dry etchingprocesses selective to the material of the intrinsic base layer 22 andthe dielectric materials of the trench isolation regions 12, 13 and thehardmask 28. The profile of the lateral extensions 44, 45 of trenches36, 38 in active device region 14 may be adjusted to have a specificshape, undercutting angle, undercut distance (i.e., bias), etc. byselecting factors such as the chemistry, duration, etc. of the etchingprocess. The etching process may be combined with implantation damage tothe semiconductor material and/or doping of the semiconductor materialto alter etch rates and, thereby, the profile. The etching process mayfurther rely on wafer orientation and anisotropic etching processes thatexhibit different etch rates for different crystallographic directions(as specified, for example, by Miller indices) in a single-crystalsemiconductor material.

The lateral extensions 44, 45 of trenches 36, 38 may have a differentprofile than apparent in the representative embodiment of FIG. 4. Forexample and with reference to FIG. 4A, the etching process may producelateral extensions 44, 45 that exhibit a box-like or square shapebecause the sidewalls 40, 42 are displaced inwardly but remain parallelwith sidewalls 41, 43. As other examples and with reference to FIGS. 4B,4C, the etching process may produce sidewalls 40, 42 in the activedevice region 14 of the substrate 10 that are faceted or beveled withmultiple sides and adjacent sides joining at corners. The sidewalls 40,42 in the lateral extensions 44, 45 may be characterized asnon-rectangular polygons, such as triangular-shaped (FIG. 4B) ordiamond-shaped (FIG. 4C).

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 4 and at a subsequent fabrication stage of theprocessing method, trench isolation regions 52, 53 comprised of anelectrical insulator are formed in the trenches 36, 38. To that end, adielectric layer 46 is deposited that fills the trenches 36, 38 withrespective portions of the electrical insulator. Dielectric layer 46 maybe deposited by a directional or non-conformal technique such that airgaps 58, 59 are formed in the lateral extensions 44, 45 between thesloped sidewalls 40, 42 and the bottom side of the intrinsic base layer22. The dielectric material in the trenches 36, 38 closes the lateralextensions 44, 45 to define the air gaps 58, 59. The air gaps 58, 59each represent empty spaces devoid of dielectric material. The trenchisolation regions 52, 53 and air gaps 58, 59 surround the collector 18and define secondary trench isolation for the active device region 14.

The trench isolation regions 52, 53 and air gaps 58, 59 collectivelyreproduce the geometrical shape of the trenches 36, 38 and are nominallyidentical to each other. Trench isolation region 52 includes sidewalls54, 55 and the air gap 58 is disposed between the sidewall 54 and theactive device region 14. Similarly, trench isolation region 53 includessidewalls 56, 57 and the air gap 59 is disposed between the sidewall 56and the active device region 14. The trench isolation regions 52, 53 andair gaps 58, 59 surround the collector 18 and define secondary trenchisolation for the active device region 14. The trench isolation regions52, 53 reside in removed sections of the trench isolation regions 12, 13and are at a shallower depth relative to the top surface 10 a than thetrench isolation regions 12, 13.

In one embodiment, the dielectric layer 46 may be deposited byplasma-enhanced CVD (PECVD) and, in a more specific embodiment, may becomprised of an oxide of silicon (e.g., SiO₂) deposited by PECVD. Theair gaps 58, 59 may have an effective dielectric constant of near unity(about 1.0) and may be filled by air at or near atmospheric pressure,filled by another gas at or near atmospheric pressure, or contain air orgas at a sub-atmospheric pressure (e.g., a partial vacuum). In oneembodiment, the composite dielectric constant of the dielectric materialcomprising trench isolation regions 52, 53 and the air gaps 58, 59 maybe less than the dielectric constant of the dielectric materialcomprising trench isolation regions 12, 13.

The trench isolation regions 52, 53 include dielectric material atlocations proximate to the corners at the juncture of the top surface 10a of the substrate 10 in the active device region 14 and the topsurfaces 12 a, 13 a of the trench isolation regions 12, 13 that wouldotherwise host divots 15. As discussed above, the divots 15 in thetrench isolation regions 12, 13 are removed when the trenches 36, 38 areformed. The dielectric material of the trench isolation regions 52, 53replaces the divots 15, which improves device planarity and covers thecorners of the active device region 14. The elimination of the divots 15and their replacement with dielectric material may decrease deviceleakage current and, thereby, may lengthen the life of the bipolarjunction transistor 70 (FIG. 7).

The dielectric layer 46 may comprise an organic or inorganic dielectricmaterial, which may be an electrical insulator characterized by anelectrical resistivity at room temperature of greater than 10¹⁰ (Ω-m).Candidate inorganic dielectric materials for dielectric layer 46 mayinclude, but are not limited to, silicon dioxide (SiO₂), fluorine-dopedsilicon glass (FSG), and combinations of these dielectric materials.Alternatively, dielectric layer 46 may comprise a low-k dielectricmaterial characterized by a relative permittivity or dielectric constantsmaller than the SiO₂ dielectric constant of approximately 3.9.Candidate low-k dielectric materials for dielectric layer 46 include,but are not limited to, spun-on organic low-k dielectrics (e.g., spun-onpolymer resins) and inorganic low-k dielectrics (e.g., organosilicateglasses, hydrogen-enriched silicon oxycarbide (SiCOH), and carbon-dopedoxides), and combinations of these and other organic and inorganicdielectrics.

With reference to FIG. 5A in which like reference numerals refer to likefeatures in FIG. 5 and in accordance with an alternative embodiment, thedielectric layer 46 may fill the lateral extensions 44, 45 of trenches36, 38 such that air gaps 58, 59 (FIG. 5) are absent and the trenchisolation regions 52, 53 are enlarged. In particular, the trenchisolation regions 52, 53 include respective portions 47, 48 of theelectrical insulator from dielectric layer 46 in the lateral extensions44, 45 between the sloped sidewalls 40, 42 of trenches 36, 38 and thebottom side of the intrinsic base layer 22. The sidewall 54 of trenchisolation region 52 is shifted to coincide and be coextensive with thesidewall 40 and sidewall 56 of trench isolation region 53 is shifted tocoincide and be coextensive with the sidewall 42. The trench isolationregions 52, 53 are partially inside the trench isolation regions 12, 13and partially inside the active device region 14 of substrate 10. Thetrench isolation regions 52, 53 and dielectric-filled portions 47, 48surround the collector 18 and define secondary trench isolation for theactive device region 14.

To provide the portions 47, 48 of electrical insulator, the dielectriclayer 46 may be deposited by a conformal deposition technique. In oneembodiment, the conformal deposition technique used to depositdielectric layer 46 may be low pressure chemical vapor phase deposition(LPCVD) using a silicon source of either silane or a mixture of silanewith nitrogen. In a specific embodiment, the dielectric layer 46 may becomprised of an oxide of silicon (e.g., SiO₂) deposited by LPCVD. In oneembodiment, the dielectric constant of the dielectric materialcomprising trench isolation regions 52, 53 may be lower than thedielectric constant of the dielectric material of trench isolationregions 12, 13.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage of theprocessing method, the dielectric layer 46 is removed from hardmask 28,but not from within the trenches 36, 38, by a wet etching process, a dryetching process, polishing, or a combination thereof. If the dielectriclayer 46 is comprised of an oxide of silicon, then RIE or an oxide etch(e.g., buffered hydrofluoric acid or diluted hydrofluoric acid) may beemployed to remove the dielectric layer 46 selective to the hardmask 28and spacers 50, 51.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 6 and at a subsequent fabrication stage of theprocessing method, the hardmask 28 is removed by a wet etching process,a dry etching process, or a combination thereof. A dielectric layer 60is applied and patterned to leave a pad at the intended location for aconnection between an emitter and the intrinsic base layer 22. Anextrinsic base layer 62 is formed on the top surface 22 a of theintrinsic base layer 22. A dielectric layer 64 is then formed on a topsurface of the extrinsic base layer 62.

In one embodiment, the extrinsic base layer 62 may be comprised ofpolycrystalline semiconductor material (e.g., polysilicon orpolycrystalline SiGe) deposited by CVD process. If the extrinsic baselayer 62 is comprised of SiGe, the concentration of Ge may have a gradedor an abrupt profile and may include additional layers, such as a Si caplayer and/or a Si bottom layer. The extrinsic base layer 62 may be insitu doped with a concentration of a dopant, such as an impurity speciesfrom Group III of the Periodic Table (e.g., boron) effective to impartp-type conductivity and may be optionally doped with carbon to retardimpurity species diffusion. As a consequence of the crystalline state ofthe single crystal section 24 of intrinsic base layer 22, the extrinsicbase layer 62 may be comprised of single crystal semiconductor materialthat relies on the semiconductor material of the intrinsic base layer 22as a growth seed. Dielectric layer 64 may be comprised of an electricalinsulator, such as SiO₂ deposited using CVD.

With reference to FIG. 8 in which like reference numerals refer to likefeatures in FIG. 7 and at a subsequent fabrication stage of theprocessing method, dielectric layer 64 is patterned usingphotolithography and etching processes to define an emitter openingaligned with the collector 18. To that end, a patterned etch mask (notshown) is applied to the dielectric layer 64. In one embodiment, theetch mask may be a photoresist layer comprised of a sacrificial organicmaterial applied by spin coating to dielectric layer 64. Thephotolithography process that patterns the photoresist layer exposes thephotoresist to radiation imaged through a photomask and develops theresulting latent feature pattern in the exposed photoresist to define awindow at the intended location for the emitter opening. The etchingprocess relies on an etch chemistry effective to remove the material ofthe dielectric layer 64 exposed within the window and stops on theextrinsic base layer 62. After the opening is formed, the etch mask isremoved. An etch mask comprised of photoresist may be removed by oxygenplasma ashing or chemical stripping.

The emitter opening is extended by an etching process, such as RIE,through the thickness of the extrinsic base layer 62 using the patterneddielectric layer 64 as a hardmask. Spacers 65, 66 are formed inside theemitter opening and extend vertically to the base of the emitteropening. The spacers 65, 66 may be formed by depositing a conformallayer comprised of an electrical insulator, such as Si₃N₄ deposited byCVD, and shaping the conformal layer with an anisotropic etchingprocess, such as RIE.

The emitter opening is extended through the pad defined by dielectriclayer 60 and an emitter 68 of the bipolar junction transistor 70 is thenformed inside the emitter opening. The emitter 68 may be formed from alayer comprised of heavily-doped semiconductor material that isdeposited and then patterned using photolithography and etchingprocesses. For example, the emitter 68 may be comprised of polysiliconor polycrystalline silicon-germanium deposited by CVD or LPCVD andheavily doped with a concentration of a dopant, such as an impuritiesspecies from Group V of the Periodic Table, such as phosphorus (P),arsenic (As), to impart n-type conductivity. The photolithographyprocess may pattern photoresist to form an etch mask that protects onlya strip of the semiconductor material registered with the emitteropening. An etching process that stops on the material of dielectriclayer 64 is selected to fashion the emitter 68 from the protected stripof semiconductor material. The etch mask is subsequently removed.

The emitter 68 is electrically and physically coupled with the intrinsicbase layer 22. The bottom part of the emitter 68 may directly contactthe top surface 22 a of intrinsic base layer 22. A head of the emitter68 protrudes out of the emitter opening and may include lateral armsthat overlap with the top surface of dielectric layer 64. A cap 69 of adielectric material may be formed on the head of the emitter 68. Thenon-conductive spacers 65, 66 encircle or surround the emitter 68 sothat the emitter 68 is electrically isolated from the extrinsic baselayer 62.

Dielectric layer 64 is lithographically patterned using the same etchmask used to form the emitter 68, and an etching process, such as RIE,with suitable etch chemistries. The etch mask is subsequently removed.The extrinsic base layer 62 and intrinsic base layer 22 are thenlithographically patterned with photolithography and etching processesand a different etch mask to respectively define an extrinsic base 72and an intrinsic base 74 of the bipolar junction transistor 70.

After patterning, the bipolar junction transistor 70 has a verticalarchitecture in which the intrinsic base 74 is located between theemitter 68 and the collector 18, and the emitter 68, the intrinsic base74, and the collector 18 are vertically arranged. The conductivity typesof the semiconductor material constituting the emitter 68 and thesemiconductor materials constituting the intrinsic base 74 are opposite.One junction 76 is defined at the interface between the emitter 68 andthe intrinsic base 74. Another junction 78 is defined at the interfacebetween the collector 18 and the intrinsic base 74. The bipolar junctiontransistor 70 may be characterized as a heterojunction bipolartransistor (HBT) if at least two of the collector 18, emitter 68, andintrinsic base 74 are comprised of differing semiconductor materials.

The bipolar junction transistor 70 includes the trench isolation regions52, 53 and air gaps 58, 59 in the undercut beneath the intrinsic base74, or the trench isolation regions 52, 53 that extend to occupy theundercut beneath the intrinsic base 74 as in FIG. 5A. Alternatively, thebipolar junction transistor 70 may include any of the trenches 36, 38shaped as in FIGS. 4A-4C, which may be completely occupied by the trenchisolation regions 52, 53 or may include gaps 58, 59 instead ofdielectric material air in the undercut beneath the intrinsic base 74.

During the front-end-of-line (FEOL) portion of the fabrication process,the device structure of the bipolar junction transistor 70 is replicatedacross at least a portion of the surface area of the substrate 10. InBiCMOS integrated circuits, complementary metal-oxide-semiconductor(CMOS) transistors (not shown) may be formed using other regions of thesubstrate 10. As a result, both bipolar and CMOS transistors may beavailable on the same substrate 10.

Standard silicidation and standard middle-end-of-line (MEOL) andback-end-of-line (BEOL) processing follows, which includes formation ofcontacts and wiring for the local interconnect structure overlying thebipolar junction transistor 70, and formation of dielectric layers, viaplugs, and wiring for an interconnect structure coupled by theinterconnect wiring with the bipolar junction transistor 70, as well asother similar contacts for additional device structures like bipolarjunction transistor 70 and CMOS transistors (not shown) included inother circuitry fabricated on the substrate 10. Other active and passivecircuit elements, such as diodes, resistors, capacitors, varactors, andinductors, may be integrated into the interconnect structure andavailable for use in the BiCMOS integrated circuit.

The trench isolation regions 52, 53 either in combination with air gaps58, 59 or in combination with dielectric-filled portions 47, 48 functionas secondary trench isolation and operate in conjunction with the trenchisolation regions 12, 13 functioning as the primary trench isolation.The trench isolation regions 52, 53 and the dielectric-filled portions47, 48 or air gaps 58, 59 may have a dielectric constant that is lessthan the dielectric constant of the dielectric material of trenchisolation regions 12, 13. The air gaps 58, 59 of unitary dielectricconstant or, alternatively, portions 47, 48 of the trench isolationregions 52, 53 are located beneath the contact region between theextrinsic base 72 and intrinsic base 74. The reduction in the dielectricconstant in the extrinsic device region beneath the intrinsic/extrinsicbase contact area and outside of the intrinsic device region may operateto improve device performance by reducing the collector-to-baseparasitic capacitance (Ccb). The unitary dielectric constant of air gaps58, 59 would provide a lower Ccb than the portions 47, 48 of dielectricmaterial in the lateral extensions 44, 45. The trench isolation regions52, 53 may eliminate possible shorting of the extrinsic base 72 to thecollector 18 because the trenches 36, 38 are initially etched throughthe shallow trench isolations 12, 13 and do not overlap with thecollector 18.

The extrinsic region of the junction 78 is accessed by initially etchingthe trenches 36, 38 into the primary trench isolation regions 12, 13 atthe edge of the active device region 14 and around the perimeter of theactive device region 14, which is accomplished without etching throughthe active device region 14 or the single crystal section 24 of theintrinsic base layer 22. The trenches 36, 38 penetrate through the fieldsection 25 of the intrinsic base layer 22 that is comprised ofpolycrystalline material. The lateral extensions 44, 45 of the trenches36, 38 are formed in a self-aligned manner relative to the trenchisolation regions 12, 13 by selectively etching the semiconductormaterial of the device region 14 (specifically, the collector 18)relative to the trench isolation regions 12, 13 and intrinsic base layer22. The trenches 36, 38 and trench isolation regions 52, 53 are formedafter the intrinsic base layer 22 is formed and before the extrinsicbase layer 62 is formed. The formation of the lateral extensions 44, 45reduces or removes the extrinsic region of the collector-base junction78 without affecting the contact area between the extrinsic base layer62 and intrinsic base layer 22, which contributes to a low baseresistance (Rb).

FIG. 9 shows a block diagram of an exemplary design flow 100 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 100 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIGS. 1-8.The design structures processed and/or generated by design flow 100 maybe encoded on machine-readable transmission or storage media to includedata and/or instructions that when executed or otherwise processed on adata processing system generate a logically, structurally, mechanically,or otherwise functionally equivalent representation of hardwarecomponents, circuits, devices, or systems. Machines include, but are notlimited to, any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g. e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g. a machine for programming a programmable gate array).

Design flow 100 may vary depending on the type of representation beingdesigned. For example, a design flow 100 for building an applicationspecific IC (ASIC) may differ from a design flow 100 for designing astandard component or from a design flow 100 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 9 illustrates multiple such design structures including an inputdesign structure 102 that is preferably processed by a design process104. Design structure 102 may be a logical simulation design structuregenerated and processed by design process 104 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 102 may also or alternatively comprise data and/or programinstructions that when processed by design process 104, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 102 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 102 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 104 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1-8. As such,design structure 102 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 104 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1-8 to generate a netlist106 which may contain design structures such as design structure 102.Netlist 106 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 106 may be synthesized using an iterative process inwhich netlist 106 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 106 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 104 may include hardware and software modules forprocessing a variety of input data structure types including netlist106. Such data structure types may reside, for example, within libraryelements 108 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 110, characterization data 112, verification data 114,design rules 116, and test data files 118 which may include input testpatterns, output test results, and other testing information. Designprocess 104 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 104 withoutdeviating from the scope and spirit of the invention. Design process 104may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 104 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 102 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 120.Design structure 120 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in an IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 102, design structure 120 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-8. In one embodiment, design structure 120may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1-8.

Design structure 120 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 120 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 1-8. Design structure120 may then proceed to a stage 122 where, for example, design structure120: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

A feature may be “connected” or “coupled” to or with another element maybe directly connected or coupled to the other element or, instead, oneor more intervening elements may be present. A feature may be “directlyconnected” or “directly coupled” to another element if interveningelements are absent. A feature may be “indirectly connected” or“indirectly coupled” to another element if at least one interveningelement is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A device structure for a bipolar junctiontransistor formed using a substrate, the device structure comprising: afirst isolation region defining a boundary of an active device region inthe substrate; an intrinsic base on the active device region of thesubstrate; a collector in the active device region; an emitter coupledby the intrinsic base with the collector; and a second isolation regionat least partially inside the first isolation region, the secondisolation region being coextensive with a portion of the first isolationregion, and the second isolation region being coextensive with a portionof the active device region, wherein the second isolation region isadjacent to the active device region, and the first isolation regionincludes a portion that separates the second isolation region from aportion of the substrate beneath the first isolation region.
 2. Thedevice structure of claim 1 wherein the first isolation region iscomprised of a first dielectric material, and the second isolationregion is comprised of a second dielectric material having a lowerdielectric constant than the first dielectric material.
 3. The devicestructure of claim 1 wherein the second isolation region extends to ashallower depth relative to a top surface of the substrate than thefirst isolation region.
 4. The device structure of claim 1 wherein theintrinsic base includes a section comprised of single crystalsemiconductor material coextensive with a portion of the substrate inthe active device region and a facet comprised of polycrystallinesemiconductor material positioned at a periphery of the section.
 5. Thedevice structure of claim 1 wherein the second isolation region does notpenetrate through the first isolation region and into the substratebeneath the first isolation region.
 6. The device structure of claim 1wherein at least two of the collector, the emitter, and the intrinsicbase are comprised of different semiconductor materials.
 7. A devicestructure for a bipolar junction transistor formed using a substrate,the device structure comprising: a first isolation region defining aboundary of an active device region in the substrate; an intrinsic baseon the active device region; a collector in the active device region; anemitter coupled by the intrinsic base with the collector; a secondisolation region at least partially inside the first isolation region,the second isolation region being coextensive with a portion of thefirst isolation region; and an air gap disposed between the secondisolation region and the active device region of the substrate, the airgap coextensive with a portion of the intrinsic base, and the air gap iscoextensive with a portion of the active device region, wherein thesecond isolation region is adjacent to the active device region, and thefirst isolation region includes a portion that separates the secondisolation region from a portion of the substrate beneath the firstisolation region.
 8. The device structure of claim 7 wherein the air gapis coextensive with a portion of the second isolation region.
 9. Thedevice structure of claim 7 wherein the first isolation region iscomprised of a first dielectric material, and the second isolationregion is comprised of a second dielectric material having a lowerdielectric constant than the first dielectric material.
 10. The devicestructure of claim 7 wherein the second isolation region extends to ashallower depth relative to a top surface of the substrate than thefirst isolation region.
 11. The device structure of claim 7 wherein theintrinsic base includes a section comprised of single crystalsemiconductor material coextensive with a portion of the substrate inthe active device region and a facet comprised of polycrystallinesemiconductor material positioned at a periphery of the section.
 12. Thedevice structure of claim 7 wherein the second isolation region does notpenetrate through the first isolation region and into the substratebeneath the first isolation region.
 13. The device structure of claim 7wherein at least two of the collector, the emitter, and the intrinsicbase are comprised of different semiconductor materials.